Freshly manufactured carbon black powder is pelletized to increase the bulk density thereof, thus making it easier to handle and more economical to ship. "Wet pelletizing" processes are most commonly employed wherein an aqueous pelletizing medium and the carbon black powder are mixed together and tumbled to form wet pellets which contain 35-60 weight percent of water. The pellets must then be dried to provide a dry, pelletized product which contains less than about 10 weight percent of water, and more commonly less than 1 weight percent thereof.
The conventional method for removing water from wet carbon black pellets is by means of an indirectly fired rotary drum dryer, since it handles the pellets very gently and is mechanically dependable. However, one of the problems associated with such dryers is the difficulty in maintaining control of the drying operation as the throughput rate of wet pellets is increased beyond certain limits, for in such cases it becomes proportionately harder to maintain a uniform temperature profile throughout the drier, or to maintain temperatures within the necessary limits, commonly 325.degree.-450.degree.F. As a consequence the pellet drying process can become a "bottleneck" when it is otherwise possible to increase the production rate in the carbon black manufacturing operation.
Efforts have been made to increase the rate at which carbon black pellets can be dried by employing fluidized bed driers since they provide high drying capacities and efficiencies relative to their size. They are also smaller physically than rotary drum dryers of equal capacity. Such efforts have been largely unsuccessful, however, since fluidized bed processes are inclined to damage the friable carbon black pellets, i.e. because of the extreme mobility of the pellets within the bed, they can be quickly broken up by repeatedly colliding with one another.